The present invention relates to the inspection of cylindrical nuclear-fuel pellets (generally based on uranium oxide), which are used in fuel rods for nuclear reactors, with a view to detecting surface defects and to assign the pellets to different classes according to the nature and the significance of the defects which they have.
FIG. 1 shows schematically a nuclear-fuel pellet of the kind used especially in fuel rods of pressurized-water reactors, formed from sintered uranium dioxide, which may contain additional elements and especially, in the case of the reuse of already irradiated fuel, a small amount of fission products and/or of plutonium oxide. The invention may also apply to consumable poison pills, such as those of gadolinium oxide.
A perfect pellet 10 has the shape of a cylinder, the periphery of which is ground and the terminal faces of which have a concave central recess and, possibly, a chamfer. The general appearance of defects liable to be encountered on the lateral face is shown schematically in FIG. 1.
A chip 12 may be caused by a shock during handling; this results in a surface recessed from the theoretical surface.
A crack 14 constitutes a zone of a width very much less than its length, the bottom of which is recessed from the theoretical surface. The combination of several cracks may constitute an ungulate defect 19.
A pit 16 constitutes a surface irregularity characterized by the presence of a ground particle set into the matrix. This particle has the same chemical nature as the pellet.
The abovementioned defects are considered three-dimensional defects. Other types of defect are known, such as, for example, spots, grinding effects and metallic inclusions. A spot is due to soiling of the pellet, arising from the method of manufacture. A grinding defect is a shinier part of the pellet due to an absence of grinding over part of the lateral surface. A metallic inclusion is a part, in general very bright, of the pellet caused by the incrustation of a foreign body in the pellet. Since this foreign body appears at the surface, it is ground and does not constitute a three-dimensional defect. All these defects described here are characterized by different shapes or colors.
Various optical methods for automatically classifying pellets have already been proposed. In particular, FR-A-2,461,944 discloses a method according to which:
each pellet to be classified is rotated about its axis,
a flat beam coming from a light source is focused onto a nominal generatrix of the pellet,
the returned light coming from an elongate zone, this zone being that illuminated on a defect-free pellet of nominal diameter, is collected,
its intensity is detected, point by point, along the elongate zone, and
the defects are deduced from the variations in the intensity.
More specifically, the defects are deduced by comparing the light intensity signal with an average value.
The foregoing method does not allow accurate evaluation of the significance of the defects and leads to rejection or retention of the pellets on the basis of relatively crude criteria. In addition, it is necessary to form two images: the first makes it possible to detect defects such as flaking, a non-ground part, a metallic inclusion; the second makes it possible to detect defects such as cracks, or pits.